[krbdev.mit.edu #2661] fake_getaddrinfo on address that doesn't reverse resolve causes crash

[Ken Raeburn]

We tracked it down, the problem is (1) Apple's getaddrinfo(dotted-quad, 
hints.flags=AI_CANONNAME) returns ai_canonname=NULL, (2) my getaddrinfo 
caching code expects ai_canonname to be non-null always, because it 
always sets AI_CANONNAME.

(2) is easier to fix....